My lab-pitt mix has been doing the head shaking think on and off for about a year. The first time it happened I rushed him to the vet...only for it to end when we got there. Once we left and were in the car he did it again, the whole staff ran out and observed it (so I was confirmed to not be nuts! lol). My vet ran tests but they all came back negative. The one thing that really indicated it wasn't too serious was that I could get his attention while he was doing it. Still it scares me to death when he does it. He had not done it in a very long time, but did it 2 weeks ago and I was FINALLY able to figure out what caused it.....heartgard. I never thought it was any preventative medication that he was taking....you just never really think about that. But luckily he did it shortly after taking the pill. He did it a couple times for the next 2 days and then stopped. Needless to say he won't be taking that medication anymore. There is something in it that is too strong for him and makes him react this way. I will probably have blood tests done to see if it is one specific ingredient but I'm not sure if that is something that I can have done locally but am looking in to it. If anyone else's dog is doing this then I would suggest talking to them while they are shaking to see if you can get their attention. My dog could shake and growl and bark all at the same time....would be funny if I wasn't so concerned.....but he can also let me hold him and kiss him until it stops. If that happens then yours may be similar to mine and it could be an ingredient.